It is already known that Streptomyces sp. KC-7367 (FERM BP-1277), a microorganism isolated from soil in Maniwa-gun, Okayama-prefecture, Japan produces two substances which are represented by the following formulae ##STR2## and exhibit a strong antifungal activity against fungi (U.S. Pat. No. 4,981,954 and EP-B-282,001).
Further, it is also reported that the 2-imino derivative of KA-7367A obtained by imination of the carbonyl group at the 2-position of the above antifungal substance KA-7367A has a high antifungal activity (U.S. Pat. No. 5,093,480 and EP-B-396,769).
However, although the above compounds KA-7367A and B exhibit an excellent antifungal activity, KA-7367A is unstable, and although the 2-imino derivative of KA-7367A is improved in stability as a result of the imination, it has a difficulty that its antifungal activity is lowered compared with its parent compound KA-7367A.
As compounds to improve these problems, EP-A -443,513 (Japanese Laid-Open Patent Publication No. 240766/1991) discloses compounds represented by the following formula ##STR3## wherein R.sub.11, R.sub.21 and R.sub.31 are the same or different with one another, and each represent a hydrogen atom, a halogen atom, an alkyl group, an alkenyl group, a haloalkyl group, an alkoxy group, an alkylthio group, an aryl or aralkyl group wherein the aromatic ring is optionally substituted by 1 to 3 substituents, or a hetero-cyclic group optionally substituted by 1 to 3 substituents, or R.sub.11 and R.sub.21 combine to form an alkylene group, Y denotes O or NOH provided that when R.sub.21 (or R.sub.11) and R.sub.31 denote hydrogen atoms simultaneously, R.sub.11 (or R.sub.21) cannot denote an n-butyl group.